In wireless communication systems of recent years, in particular, mobile communication systems of the third generation or later, a distortion compensation process may be performed by implementing a Digital PreDistortion (DPD) scheme, for the purpose of improving power efficiency of amplifiers provided in wireless communication apparatuses. Further, for example, also in multiplex wireless communication systems in which wireless communication is used in the trunk network thereof, a DPD scheme may be applied to wireless communication apparatuses provided at various types of communication stations.
The DPD scheme is a technique used for cancelling non-linear distortion occurring in an amplifier, by applying in advance, to a transmission signal, distortion having a characteristic inverse to the characteristic of the non-linear distortion occurring in the amplifier and causing the amplifier to amplify the transmission signal. The DPD scheme can be a factor that determines a maximum transmission power level of each wireless communication apparatus. As a result, the DPD scheme can have an impact on cell coverages of the wireless communication apparatuses.
Generally speaking, as a method for realizing the DPD scheme, a LookUp Table (LUT) method is known by which a lookup table is used for storing therein mutually-different distortion compensation coefficients in correspondence with power levels of transmission signals. The distortion compensation coefficients stored in the LUT correspond to characteristics that are inverse to the characteristics of non-linear distortion occurring in amplifiers. Accordingly, by reading, from the LUT, one of the distortion compensation coefficients corresponding to the power level of a transmission signal and multiplying the pre-amplification transmission signal by the read coefficient, it is possible to compensate the non-linear distortion occurring in the transmission signal at the amplifier.
Patent Literature 1: Japanese National Publication of International Patent Application No. 2016-510551
Patent Literature 2: International Publication Pamphlet No. WO 2014/050218
Patent literature 3: Japanese Laid-open Patent Publication No. 2003-304122
However, the DPD scheme using the LUT method has a problem where there is a long delay period before the distortion compensation coefficient is read from the LUT, and the communication is thus delayed.
More specifically, because the LUT has stored therein the distortion compensation coefficients in correspondence with the power levels of transmission signals, in order to read an appropriate distortion compensation coefficient, the power level of each transmission signal is calculated, and an address in the LUT is determined on the basis of the power level. After that, a distortion compensation coefficient is read from the determined address so as to multiply the transmission signal thereby. Accordingly, because the distortion compensation process for the transmission is not performed until the distortion compensation coefficient is read from the LUT, a delay occurs in the transmission signal prior to the amplification process.
In particular, in multiplex wireless communication systems, a long-distance transfer may be realized by using one or more relay stations. When a distortion compensation process using the DPD scheme is also performed in the wireless communication apparatuses provided at the relay stations, delays are accumulated before the signal reaches the final receiving station. For this reason, to keep the delay period shorter than a tolerated length, the number of relay stations provided on the way to the receiving station is limited, which makes it difficult to realize a sufficiently long transfer distance.
Further, for example, when a multiplex wireless communication system is used for an application requiring a real-time response such as that used for maintenance of an electric power transmission facility or for a system in a stock brokerage company, it is desirable to reduce the delay in the entire system by reducing the delay in the communication.